/Engine/Plugins/FX/Niagara/Source/NiagaraEditor/Public/NiagaraNodeAssignment.h
Set the assignment target and default value. This does not call RefreshFromExternalChanges, which you will need to do to update the internal graph. Returns true if values were changed.
bool SetAssignmentTarget ( int32 Idx, const [FNiagaraVariable](API\Plugins\Niagara\FNiagaraVariable) & InVar, const [FString](API\Runtime\Core\Containers\FString) * InDefaultValue )